High molecular weight linear thermoplastic polyesters such as polyethylene terephthalate and poly(1,4-butylene terephthalate) are well known film, molded article and fiber formers prepared by methods outlined in Whinfield et al, U.S. Pat. No. 2,465,319 and Pengilly, U.S. Pat. No. 3,047,539 and elsewhere. Polyethylene terephthalate has not been greatly exploited as a molding and extrusion resin because it crystallizes so slowly from the melt that three-dimensional parts molded from it, in conventional molding cycles, are brittle in thick sections. The prior art has overcome, to some extent, this problem by providing careful molecular weight control in the production of polyethylene terephthalate; by including nucleating agents to hasten crystallization; or by using unconventional, long, two-step molding cycles.
In contrast, poly(1,4-butylene terephthalate) resins crystallize very rapidly from the melt. These provide excellent molding compositions because they can be fabricated with moderate stock temperatures, low mold temperatures and rapid cycle times. These resins can also be provided in reinforced and flame retardant embodiments. Because of their highly crystalline nature, these resins are superior in chemical resistance, thermal stability and product appearance (they have a smooth, glossy finish). Such resins also have superior strength, stiffness, low friction and wear properties and good resistance to brittle fracture. Because of their rapid crystallization times the poly(1,4-butylene terephthalate) resins do not need nucleating agents during the injection molding process.
One disadvantage of the poly(1,4-butylene terephthalate) resins resides in their significantly higher cost of manufacture in comparison with poly(ethylene terephthalate) due mainly to the higher cost of the raw materials employed in its manufacture.
The art has long recognized that when the above-discussed polyalkylene terephthalate resins are blended, i.e., when polyethylene terephthalate and polybutylene terephthalate resins are physically blended, a number of the above problems can be eliminated. For instance, when high levels of polybutylene terephthalate are utilized in the blend it is not necessary to use a nucleating agent. Alternatively, compositions which have higher ratios of polyethylene terephthalate to polybutylene terephthalate can be substantially improved by the use of nucleating agents.
British Patent No. 1,060,401 discloses that, in addition to the above advantages, parts molded from the combination of the two polyalkylene terephthalates often show properties superior to those of either of the components in their pure stage.
One problem that has plagued the art, however, especially those compositions which contain any amount of polyethylene terephthalate and especially those that contain major proportions of polyethylene terephthalate, is the fact that these resin compositions must be thoroughly dried, i.e., free from as much moisture as possible, prior to being intimately admixed in an extruder and/or prior to a fabrication step. By fabrication is meant the melting, forming and solidification of an article from the resin composition. These drying operations often requires special equipment and is expensive in terms of time and energy wasted. For example, U.S. Pat. No. 3,953,394, which discusses polyester blends, suggests that it is "always very important to thoroughly free all of the ingredients . . . from as much water as possible" and teaches typically that the resin should be predried "under vacuum at 100.degree. C. for twelve hours". Typically, U.S. Pat. No. 3,619,267 to Weissermel et al which discloses the use of montan wax or montan wax ester as a nucleating agent in linear polyester compositions of terephthalic acid and saturated aliphatic or cycloaliphatic diols state that the polyester must have a moisture content of less than 0.01% by weight which necessitates a drying step prior to the nucleating agent addition.